The field is a two-stroke internal combustion opposed-piston engine. Particularly, the field concerns an air handling system with provision for transporting exhaust from the cylinders of an opposed-piston engine.
A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. One example of a two-stroke cycle engine is an opposed-piston engine in which two pistons are disposed in the bore of a cylinder for reciprocating movement in opposing directions along the central axis of the cylinder. Each piston moves between a bottom center (BC) location where it is nearest one end of the cylinder and a top center (TC) location where it is furthest from the one end. The cylinder has ports formed in the cylinder sidewall near respective BC piston locations. Each of the opposed pistons controls one of the ports, opening the port as it moves to its BC location, and closing the port as it moves from BC toward its TC location. One of the ports serves to admit charge air into the bore, the other provides passage for the products of combustion out of the bore; these are respectively termed “intake” and “exhaust” ports (in some descriptions, intake ports are referred to as “air” ports or “scavenge” ports). In a uniflow-scavenged opposed-piston engine, pressurized charge air enters a cylinder through its intake port as exhaust gas flows out of its exhaust port, thus gas flows through the cylinder in a single direction (“uniflow”)—from intake port to exhaust port.
In FIG. 1, an opposed-piston engine 10 having at least one ported cylinder 50 embodies a two-stroke-cycle internal combustion engine. Each ported cylinder 50 has a bore 52 and longitudinally spaced intake and exhaust ports 54 and 56 formed or machined in respective ends of a cylinder wall. Each of the intake and exhaust ports 54 and 56 includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid bridge. In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions shown in FIG. 1. In the example shown, the engine 10 further includes two spaced-apart crankshafts 71 and 72 disposed in a parallel array. Intake and exhaust pistons 60 and 62 are slideably disposed in the bore 52 with their end surfaces 61 and 63 opposing one another. The intake pistons 60 are coupled to the crankshaft 71, and the exhaust pistons are coupled to the crankshaft 72.
As the pistons 60 and 62 in a cylinder 50 near TC, a combustion chamber is defined in the bore 52 between the end surfaces 61 and 63 of the pistons. Combustion timing is frequently referenced to the point in a compression cycle where minimum combustion chamber volume occurs; this point is referred to as “minimum volume.” Fuel is injected directly into cylinder space located between the end surfaces 61 and 63. In some instances injection occurs at or near minimum volume; in other instances, injection may occur before minimum volume. Fuel is injected through fuel injector nozzles 70 positioned in respective openings through the sidewall of the cylinder 50. Preferably, the fuel injector nozzles 70 are positioned to inject respective sprays of fuel in opposing directions along a diameter of the bore 52. The fuel mixes with charge air admitted into the bore 52 through the intake port 54. As the air-fuel mixture is compressed between the end surfaces 61 and 63, the compressed air reaches a temperature that causes the fuel to ignite. Combustion follows.
In multi-cylinder opposed-piston engines with two crankshafts, the crankshafts are configured in such a manner as to cause minimum volume conditions (MV) to occur among the cylinders in a predetermined order during each complete revolution of a designated one of the crankshafts. For example, in a four-cylinder engine, with the cylinders numbered 1-4 in linear sequence, one order may have cylinder 2 achieving MV at 90°, cylinder 4 at 180°, cylinder 1 at 270°, and cylinder 3 at 360°. Fuel injection operation is timed to synchronize ignition and combustion with the MV order. Each combustion occurrence is termed a “cylinder firing”, and the engine is considered to have a “cylinder firing sequence” (also referred to as a “cylinder firing order”) that conforms to the MV (or injection) sequence.
As per FIG. 1, the engine 10 includes an air handling system 80 comprising a turbocharger 120 with a turbine 121 and a compressor 122 that rotate on a common shaft 123. The turbine 121 is in fluid communication with an exhaust channel 124 and the compressor 122 is in fluid communication with a charge air channel 126. Exhaust gas discharged from the exhaust ports 56 is received by an exhaust gas collector 125 and flows from there into the exhaust channel 124. The turbine 121 is rotated by the fluid pressure of the exhaust gas passing through it. This rotates the compressor 122, causing it to generate charge air by compressing fresh air. The charge air produced by the compressor 122 flows through the charge air channel 126 to a cooler 127 from where it is pumped by a supercharger 110 to the intake ports. Charge air compressed by the supercharger 110 can be output through a cooler 129 to an intake distributor 130. The intake ports 54 receive pressurized charge air via the intake distributor 130.
Many applications require integration of a two-stroke cycle, opposed-piston engine into engine space designed for traditional four-stroke engines, and it is critical to minimize the size of the engine without sacrificing engine efficiency and performance characteristics. Location, layout, and integration of charge air and exhaust subsystems pose significant challenges when designing compact opposed-piston engine constructions for fitment into traditional motor spaces. For example, see commonly-owned U.S. patent application Ser. No. 13/891,466, titled “Placement of an Opposed-Piston Engine in a Heavy-Duty Truck”.
Related application Ser. Nos. 14/284,058 and 14/284/134 describe how some of these compactness challenges are met in an opposed-piston engine construction with multiple cylinders arranged inline in a cylinder block. A single exhaust chamber inside the cylinder block contains all of the cylinder exhaust ports, thereby eliminating the need for a flanged, multi-pipe manifold construction. Instead of collecting and transporting exhaust gas discharged from individual exhaust ports with dedicated sets of pipes, exhaust gas discharged by all of the exhaust ports is collected in the single exhaust chamber. The discharged exhaust gas exits the chamber through at least one exhaust outlet that opens through the cylinder block. Advantageously, only a single pipe is required to transport exhaust gas from the exhaust outlet, thereby eliminating flange-to-flange spacing between separate pipes of conventional exhaust manifolds. As a result, the weight of multi-pipe manifolds is eliminated, inter-cylinder spacing can be reduced, and the engine can be made more compact.
The combination of open exhaust chamber construction with the elimination of multi-pipe exhaust manifolds reduces spikes, surges, oscillations, and other asymmetries in the flow of exhaust from the exhaust ports to the turbocharger. The smooth exhaust flow contributes to consistent, reliable combustion and scavenging. However, the in-line construction of the opposed-piston engine can impose significant constraints on engine packaging space that prevent close, smooth coupling between the exhaust chamber and the turbocharger, which can limit the benefits achievable with the open exhaust chamber construction. For example, exhaust outlets that open through opposing sides of an inline cylinder block can provide straight, short, smooth transport paths from the open exhaust chamber. But without packaging space for straight, short channels along the sides of the cylinder block, the transport paths to the turbocharger can become eccentric, thereby introducing turbulence into the exhaust flow.
Thus, there is a need for further improvement in turbocharged, uniflow-scavenged, opposed-piston engines in order to further contribute to consistent, reliable combustion and smooth gas flow with exhaust constructions that preserve or advance the goal of compactness.